Screen grid electron discharge tube



Sept..l4, 1937. i JOBST 2,092,804

SCREEN GRID ELECTRON DISCHARGE TUBE I Filed Oct. 22, 1936 6 a 5 FE a w M I,

0/ 1%. L 1 I \I L i L -2/ INVENTOR L 1 GUENTHER JOBST L 1 B M, ATTORNEY Patented Sept. 14, 1937 warren s'rA'rss I 2,092,804 SCREEN can) ELECTRON Discusses TUBE Giinther Jobst, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b. H., Berlin, Germany, a corpora- I tion of Germany Application October 22, 1936,-Serial No. 106,936 In Germany November '7, 1935 3 Claims.

This invention relates to electron discharge tubes, and more particularly to means for preventing the passage of secondary-electrons from a secondary electron emitting electrode to an adjacent positive electrode. Means known in the art for this purpose in screen grid tubes, for example, include grid-like electrodes having conductors impressed with either fixed or variable potentials to produce between the screen grid and the anode a potential threshold or level that cannot be passed by the secondary electrons.

More careful consideration shows that for this purpose it is not really necessary to use such a conductor or conducting electrode. It has been proposed to utilize for the production of such potential thresholds perforated diaphragms or shields consisting of insulation and mounted in the tube with no leads or connections of any kind to the outside or to other electrodes, so that electrically the shield floats in the tube. However, in connection with such floating shields of insulation two factors must be noted; there may occur, through leakage or conduction from the adjacent positive electrodes, due to faulty insulation of the supporting means common to all the electrodes and the like, a charging to a positive potential of the perforated shield or diaphragm of insulation, which defeats the purpose of the diaphragm; and for high frequency amplification, the use of non-conducting perforated shields or diaphragms has a further disadvantage, as the shield structure takes on alternating potentials due to self capacity and thus in turn influences the electron discharge in an undesirable manner.

Giving due weight to these factors and with the limitation that the invention is to be used in the low frequency field, the object of the present invention is to provide novel means for preventing the flow of secondary electrons from a secondary electron emitting electrode to a more positive electrode. The invention utilizes for that purpose diaphragms or shields of substantially non-conducting material, which are fully sufilcient for the production of potential thresholds,

' particularly for the prevention of the passage of secondary electrons between positive adjacent electrodes, provided that, in accordance with the invention, care is taken that their margin or edge potentials are kept below the cathode potential preferably by metallic rims or borders disposed on the shields or diaphragms outside the path of the electron discharge, whereby a charging of the insulation to positive potential is avoided. Such shields or diaphragms may often be of the general form of the usual grid electrodes, and may, for instance, consist of mesh made of insulation which, if necessary, may for mechanical reasons, have a metal core; or. may also be made of suitably formed ceramic sheets or cylinders provided with Very large holes and made of pressed mica pieces or sheets. Insulating shields of this type may be used for constructional reasons, in many cases more advantageously than metallic conductors. Thus it is feasible for instance, in the case of a cylindrical tube structure to utilize the cylindrical tubular shield or diaphragm of insulation also as a support in the electrode assembly.

Charging of the insulation by impinging ions is very improbable, due to the extremely high vacuum generally used today. In order, however, to prevent the formation on the non-conductor shield of localized centers which become temporarily positive due to the appearance of positive ions and thus might become adapted to supply upon the impingement of primary electrons more secondary electrons than primary electrons that have arrived, resulting in an increase of the positive charge of the diaphragm, the surface of the shield or of the conductive rim may be so formed or treated, for example, by coating with soot or carbon, that its specific secondary emission, that is the ratio of secondary electrons emitted to primary impinging electrons, is less than unity, or smaller than 1, within the operating potential range of the tube.

For a better understanding of my invention, reference may be made to the accompanying drawing in which Figure 1 is a longitudinal section of one embodiment of my invention in a cylindrical structure; Figure 2 is a plan View of the insulating diaphragm or shield before it is rolled up into tubular formyand Figure 3 is a fragmentary View of a portion of a mesh form of insulating shield.

The tube shown in Figure 1 is of the screen grid type, and comprises a highly evacuated envelope H3 with the usual stem H, and enclosing an electrode assembly mounted on the stem.

The electrode assembly comprises the conventional pair of insulating spacers l2 and I3, and

the conventional indirectly heated cathode l4 control grid l5, screen grid l6, which is kept positive and to the extent that it collects electrons is a grid-like anode, and conventional tubular anode l1 surrounding and coaxial with the screen grid and constituting a second or main anode.

Interposed between the screen grid I6 and the anode I1 is a perforated diaphragm or shield 20 cathode.

with a surface of insulation and preferably made of insulating material, such as sheet mica, and having comparatively large perforations or openings 2|. In accordance with the invention the diaphragm 20 has metal members, such as marginal rims or borders 22 of metal which are outside of the path of discharge from the cathode through the grids to the anode and, therefore, out of the discharge. The metal parts of the shield or diaphragm are kept at a potential as least as low as that of the cathode l4, preferably through direct electrical connections, such as the leads 23, to the cathode. The leads 23 are preferably inside the envelope, although it is obvious that the connection to the cathode can be made outside the tube if desired. Maintaining the marginal potential of the shield or diaphragm equal to or less than the potential of the cathode prevents charging up of the shield or diaphragm.

The shield is preferably made of a sheet of mica, as shown in Figure 2, although other ceramic materials may be used. When in tubular form, as shown in Figure 1, the shield has sufficient mechanical strength so that it may be used as a support member in the electrode assembly for holding the insulating spacers I2 and I3 in proper relation to each other.

The insulating shield or diaphragm may, as shown in Figure 3, be made in the form of a mesh or netting, preferably of metal wires 24, coated with insulation 25, and connected to the In operation the tube is connected in the conventional way for screen grid tubes to a battery 26 with an input circuit 21 connected to the control grid l6, and a load circuit 28 to the second or main anode I1.

I claim:'

1. An electron discharge device comprising an electron emitting cathode, a grid-like anode surrounding said cathode, a second anode surrounding said grid-like anode, and a perforated diaphragm of insulation interposed between said anodes and electrically connected within said device to said cathode.

2. An electron discharge device comprising an electron emitting cathode, a grid-like anode surrounding said cathode, a second anode surrounding said grid-like anode, and a perforated diaphragm of insulation interposed between said anodes andhaving outside the path of discharge from said cathode to said second anode a metal rim electrically connected Within said device to said cathode. a

3. An electron discharge tube of the screen grid type adapted for low frequency amplification and comprising a cathode,-a screen grid electrode, an anode surrounding said screen grid, and a perforated diaphragm of insulation interposed in the path of discharge through said screen grid to said anode, said diaphragm having metal members out of said path of discharge and electrically connected within said device to said cathode.

GIJNTHER JOBST. 

